Answer:
A, because of the cataracts.
Explanation:
The cataracts made the river too dangerous to travel in as the rock filled rapids would destroy the boat, and destroy them.
Roman concrete is very similar to what we use today. It was used to build structures and many buildings. It was revolutionary at the time as not many things were made of something such a concrete. The Romans proved to be much more advanced than other people and a leader in the world at the time.
I believe the correct answer would be Egypt
